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Subject[PATCH kcsan 05/32] kcsan: Address missing case with KCSAN_REPORT_VALUE_CHANGE_ONLY
Date
From: Marco Elver <elver@google.com>

Even with KCSAN_REPORT_VALUE_CHANGE_ONLY, KCSAN still reports data
races between reads and watchpointed writes, even if the writes wrote
values already present. This commit causes KCSAN to unconditionally
skip reporting in this case.

Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
kernel/kcsan/report.c | 27 ++++++++++++++++++++-------
1 file changed, 20 insertions(+), 7 deletions(-)

diff --git a/kernel/kcsan/report.c b/kernel/kcsan/report.c
index 33bdf8b..7cd3428 100644
--- a/kernel/kcsan/report.c
+++ b/kernel/kcsan/report.c
@@ -130,12 +130,25 @@ static bool rate_limit_report(unsigned long frame1, unsigned long frame2)
* Special rules to skip reporting.
*/
static bool
-skip_report(int access_type, bool value_change, unsigned long top_frame)
+skip_report(bool value_change, unsigned long top_frame)
{
- const bool is_write = (access_type & KCSAN_ACCESS_WRITE) != 0;
-
- if (IS_ENABLED(CONFIG_KCSAN_REPORT_VALUE_CHANGE_ONLY) && is_write &&
- !value_change) {
+ /*
+ * The first call to skip_report always has value_change==true, since we
+ * cannot know the value written of an instrumented access. For the 2nd
+ * call there are 6 cases with CONFIG_KCSAN_REPORT_VALUE_CHANGE_ONLY:
+ *
+ * 1. read watchpoint, conflicting write (value_change==true): report;
+ * 2. read watchpoint, conflicting write (value_change==false): skip;
+ * 3. write watchpoint, conflicting write (value_change==true): report;
+ * 4. write watchpoint, conflicting write (value_change==false): skip;
+ * 5. write watchpoint, conflicting read (value_change==false): skip;
+ * 6. write watchpoint, conflicting read (value_change==true): impossible;
+ *
+ * Cases 1-4 are intuitive and expected; case 5 ensures we do not report
+ * data races where the write may have rewritten the same value; and
+ * case 6 is simply impossible.
+ */
+ if (IS_ENABLED(CONFIG_KCSAN_REPORT_VALUE_CHANGE_ONLY) && !value_change) {
/*
* The access is a write, but the data value did not change.
*
@@ -228,7 +241,7 @@ static bool print_report(const volatile void *ptr, size_t size, int access_type,
/*
* Must check report filter rules before starting to print.
*/
- if (skip_report(access_type, true, stack_entries[skipnr]))
+ if (skip_report(true, stack_entries[skipnr]))
return false;

if (type == KCSAN_REPORT_RACE_SIGNAL) {
@@ -237,7 +250,7 @@ static bool print_report(const volatile void *ptr, size_t size, int access_type,
other_frame = other_info.stack_entries[other_skipnr];

/* @value_change is only known for the other thread */
- if (skip_report(other_info.access_type, value_change, other_frame))
+ if (skip_report(value_change, other_frame))
return false;
}

--
2.9.5
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